Naphthalene (NA) is a ubiquitous pollutant to which humans are widely exposed. NA causes nasal and lung toxicities, including tumors, in adult rats and mice and has been classified as a possible human carcinogen. The mechanism of NA carcinogenicity, which may involve both genotoxic and non-genotoxic events, is not clear. A prerequisite for NA cytotoxicity is bioactivation by cytochrome P450 (CYP) enzymes. The reactive metabolites formed, which derive from the NA-epoxide (NAO), can deplete cellular glutathione and bind covalently to proteins. Research in the current funding cycle, which was focused on NA bioactivation and acute lung toxicity, provided compelling evidence for the ability of human CYP2A13 and 2F1 to mediate NA?s lung toxicity in vivo in a humanized mouse model, and insights on the interplay between systemic disposition and target tissue bioactivation of inhaled NA and its impact on NA?s airway toxicity. Initial novel evidence was also obtained for the ability of NA to produce stable DNA adducts ex vivo, and for a possible role of systemically generated NA metabolites in lung toxicity in vivo. In the proposed studies for the next funding cycle, we will continue to study the metabolic mechanisms of NA lung toxicity by: identifying liver-generated NA metabolites that contribute to lung toxicity in vivo (Aim 1), determining the ability of human CYP2A6 expressed in the mouse liver to mediate airway toxicity of inhaled NA (Aim 2), and identifying the stable NA- DNA adducts in the lung and dissecting metabolic mechanisms of their formation (Aim 3). The central hypothesis is that NA has the potential to cause both cytotoxicity and genotoxicity in human lung, and that the metabolism of NA in both lung and liver influences the toxic outcome on an individual basis. We will employ a combination of in vivo, ex vivo, and in vitro approaches, and utilize novel genetically modified or humanized mouse models, as well as human lung cells and liver microsomes, to address the specific aims. The long- term goal of these studies is to define the metabolic mechanisms that influence NA-mediated lung toxicity in experimental animals and humans. The outcome is expected to improve assessment of human lung disease risks from exposures to NA and other related chemicals.